Water based ink for ink jet recording and ink jet recording method

ABSTRACT

A water-based ink for ink-jet recording enables the expression of sharp text and lines in high density black, and the formation of a high quality gradation image in which the graininess is reduced in a black image of medium to high lightness. The inks include a color ink (A) and a color ink (B) each of which independently has a lightness index L* according to the L* a* b* calorimetric system of about 60 or less. The color ink (A) and the color ink (B) satisfy the following expression (I):
 
about 120°≦|α−β| about ≦240°  (I)
 
wherein α is the hue angle of the color ink (A) and β is the hue angle of the color ink (B). The chroma C* of a hue obtained by the combination of the color ink (A) and the color ink (B) is about 30 or less.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Application No. 2005-118999, filed Apr. 15, 2005, the disclosure of which is incorporated by reference.

BACKGROUND

In ink-jet recording methods, ink droplets are generally formed by using any of various ink discharge methods. Such ink-jet recording methods include applying a high voltage to form an electrostatic attraction or applying mechanical vibration or displacement to an ink using a piezoelectric element. Another ink-jet recording method heats an ink to generate bubbles and utilizes the resulting pressure. These methods provide ink droplets and force the ink droplets to fly and subsequently adhere as ink dots to a recording material, such as a recording paper, thereby performing the recording. Ink jet recording methods typically generate less noise and allow high-speed printing or multi-color printing.

Ink-jet recording methods can be used to generate images on various types of recording material, for example plain paper, dedicated recording paper; and optical transparency sheets for a slide or overhead projector (OHP). Other ink jet recording methods utilize a color-separation plate for forming a positive plate for color printing; or a color mosaic filter (CMF) for a color display device such as a liquid crystal device; and the like.

When a color image is formed by the ink-jet recording method, various colors are generally developed using a subtractive color mixing method by overprinting inks of three primary colors (yellow (Y), magenta (M) and cyan (C)), on one another in an appropriate combination on the same pixel. For example, red (R), green (G), blue (B) and black (BK), can be generated by mixing the combinations of primary colors as shown in the following expressions: Y+M→R Y+C→G M+C→B Y+M+C→BK

Red (R) can be developed by appropriately mixing yellow (Y) and magenta (M), green (G) can be developed by appropriately mixing yellow (Y) and cyan (C), blue (B) can be developed by appropriately mixing magenta (M) and cyan (C), and black (BK) can be developed by appropriately mixing yellow (Y), magenta (M) and cyan (C). However, when a black image is formed by overprinting inks of three primary colors (yellow, magenta and cyan), on one another, the volume of the ink droplets on a recording material becomes large. The line becomes thicker compared with portions where colors other than black are recorded resulting in an unnatural image, and ink absorption failure of the recording material occurs resulting in lowering of the drying property and the like.

In many cases, text and lines are formed in black, thus high optical density, sharpness and lightfastness are required for the black text and black lines. Often, therefore, a black ink is added to an ink set for ink-jet recording in addition to color inks of yellow, magenta and cyan.

A black dye or a black pigment such as carbon black often is used as a coloring agent for black ink. However, a black ink containing carbon black as a coloring agent has a problem in that the fixing property of the ink is poor, especially when printing on a glossy paper or on a glossy texture.

Since black ink has a high optical density per se, when a black image is formed only with a black ink, it is often difficult to develop an image with gradation in an area of a medium lightness to a high lightness. It is particularly difficult to reduce the graininess in a black image in an area of a high lightness.

Generally, “a black image of a medium lightness to a high lightness” means an image in dark gray to light gray and specifically, an area with a lightness (L*) of about 30 or more and about 80 or less and with a chroma (C*) of about 50 or less.

In response to the problem of the image quality of such a black image of a medium lightness to a high lightness, a method using plural black inks having different densities as black inks has been proposed (U.S. Pat. No. 6,596,065). However, the use of plural black inks having different densities increases the number of inks constituting an ink set, resulting in increased cost and increased size of an ink-jet recording apparatus.

SUMMARY

Two water-based color inks for ink-jet recording having a color relationship such that the two color inks form a black image with gradation in an area of low lightness to high lightness.

DETAILED DESCRIPTION General Overview

Aspects of the invention allow the expression of sharp text and lines in high density black, enable the formation of a high quality gradation image in which the graininess is reduced in a black image of a medium lightness to a high lightness (e.g., an achromatic image in dark gray to light gray) in ink-jet recording, reduce the number of inks needed in an ink combination in such a case, and obtain an excellent drying properties when printing is performed on a glossy paper.

When two color inks having a lightness index (L*) of not higher than a specified value and a color relationship are used, a black image with gradation in an area of a low lightness to a high lightness can be formed and graininess reduced. Graininess is particularly problematic in areas of medium to high lightness. Moreover, since a black image is formed by overprinting two color inks on each other, the volume of the ink droplets on a recording material is reduced compared with forming a black image by overprinting inks of three primary colors (yellow, magenta and cyan), on one another. The drying property of the ink is also improved, and blurring of text and lines is reduced.

Aspects of the invention include a water-based ink combination for ink-jet recording comprising a color ink (A) and a color ink (B) each of which independently has a lightness index (L*) according to the L* a* b* calorimetric system of about 60 or less, respectively, wherein the color ink (A) and the color ink (B) satisfy a color relationship of the following expression (I): about 120°≦|α−β|≦ about 240°  (I) wherein α is the hue angle of the color ink (A) and β is the hue angle of the color ink (B). The chroma (C*) of a hue obtained by the combination of the color ink (A) and the color ink (B) is about 30 or less.

Further aspects utilize a color ink (A) and a color ink (B) satisfying a color relationship of expression (II): about 150°≦|α−β|≦ about 220°.

Further, aspects of the invention include an ink-jet recording method for forming a black image of a low lightness to a high lightness by printing the image with the above-mentioned water-based ink combination for ink-jet recording such that a color ink (A) and a color ink (B) are overprinted on each other.

By overprinting two color inks (A) and (B) having the above-mentioned color relationship on each other, a black image of a low lightness to a high lightness with improved gradation can be formed. Also, graininess that is particularly problematic in areas of medium to high lightness can be reduced. Accordingly, a gray gradation image of high quality can be formed.

Further, since these color inks (A) and (B) have a lightness index (L*) according to the L* a* b* colorimetric system of about 60 or less, it is possible to form text, lines, and the like that require high density black clearly and with high density by overprinting the color inks (A) and (B) on each other. Therefore, it is possible to avoid the need for a black ink in an ink combination.

Further, the color ink (A) that is one of the two color inks (A) and (B) having the above-mentioned color relationship may be any of a yellow ink, a magenta ink and a cyan ink. A full-color image can be formed using four colors, in which the color ink (B) is added to a yellow ink, a magenta ink and a cyan ink. Therefore, a medium color can be favorably developed, and in particular, the range of color reproduction in the vicinity of the expressible color range can be broadened by the color ink (B).

In another aspect, a color ink (A) and a color ink (B) having the above-mentioned color relationship can be combined to form an ink set.

In another aspect, an ink-jet recording apparatus has ink cartridge receivers for receiving designated ink cartridges and nozzles for application of the inks to a recording material. The ink cartridge receivers receive at least a color ink (A) and a color ink (B) having the color relationship described above.

Illustrative Aspects of the Invention

Lightness index (L*), hue angle (h) and chroma (C*) are based on the L* a* b* colorimetric system standardized by the Commission International de l'Eclairage (CIE) in 1976, which is defined in JIS Z 8729 in the Japanese Industrial Standards (JIS).

In accordance with one aspect of the invention, an ink combination for ink-jet recording comprises two inks which are a color ink (A) and a color ink (B) having a lightness index (L*) according to the L* a* b* colorimetric system of about 60 or less, respectively, and satisfy a color relationship of the following expression (I): about 120°≦|α−β|≦ about 240°  (I) wherein α is the hue angle of the color ink (A) and β is the hue angle of the color ink (B). The chroma (C*) of a hue obtained by the combination of the inks (A) and (B) is about 30 or less.

Further aspects utilize a color ink (A) and a color ink (B) satisfying a color relationship of expression (II): about 150°≦|α−β|≦ about 220°.

In general, complementary colors are positioned opposite each other on the hue circle and defined as two colors which develop an achromatic color when they are mixed. In fact, whether or not the two color inks develop an achromatic color when they are mixed in ink-jet recording depends not only on the hue angle, but also on the concentration of the coloring agent, the volume of the ink droplets, the viscosity of the ink, the permeability of the ink and the like. It was discovered that when the difference between the hue angles of the two color inks is about 120° to about 240°, preferably about 150° to about 220°, a color considered to be an achromatic color can be developed by mixing them. Therefore, color inks (A) and (B) satisfying the expression (I) described above are used.

Further, since the color ink (A) and the color ink (B) are included in the ink combination for forming a black images of a low lightness to a high lightness (e.g., an achromatic image in black to white), text and lines that require a high optical density, sharpness and lightfastness can also be formed. Therefore, the lightness indices (L*) of the two color inks (A) and (B) are set to about 60 or less, respectively. In this way, it is possible to avoid the need for a conventional black ink in an ink combination.

For the formation of black images of high optical density, lower lightness indices (L*) of the color ink (A) and the color ink (B) generally work better. However, in order to also enable the formation of full-color images by appropriately using the color ink (A) and the color ink (B), their lightness indices (L*) are preferably each set to about 30 or more.

The value of the hue angle of each ink can be obtained by measuring the color with a spectrocolorimeter for a printed material obtained by printing in ink on a glossy paper with an inkjet printer so as to cover 100% of the area of 1200×1200 dpi resolution (e.g., solid printing). In this case, the glossy paper is a paper in which a coated layer for giving surface smoothness is provided on a base paper, and specific examples include a dedicated paper “glossy photo paper, BP60GLA”, manufactured by Brother Industries, Ltd., “Kassai (registered trademark) glossy finish” manufactured by Fuji Photo Film Co., Ltd., an inkjet printer paper (glossy paper) manufactured by Kokuyo Co., Ltd., a thick glossy paper manufactured by Kodak Co., Ltd. and the like.

Examples of the inkjet printer that can be used in solid printing for printed materials include a multifunction printer with inkjet printer, MFC-3100C, manufactured by Brother Industries, Ltd. and the like. Examples of the spectrocolorimeter that can be used include SC-T manufactured by Suga Test Instruments Co., Ltd., Spectrolino manufactured by Gretag Macbeth and the like.

The hue angle (h) can be measured, for example, with a light source set to D₆₅ and a viewing angle set to 2°.

Any suitable colors may be combined for the color ink (A) and the color ink (B) satisfying the color relationship of expression (I) above. For example, the color ink (A) may be any of a yellow ink, a magenta ink and a cyan ink. Accordingly, a four-component ink combination could include a yellow ink, a magenta ink and a cyan ink so that a full-color image can be formed, and a color ink (B) having the above-mentioned color relationship to any of these colors. In this way, it is possible to favorably form full-color images by appropriately combining four color inks of this ink combination. In addition, black images of low to high lightness can be formed by using two inks having a color relationship satisfying expression (I).

When referring to suitable colors, variations of the colors are contemplated. For example, one skilled in the art understands that yellow can be any of a spectrum of yellow, from dark to light, bright to dull, orangeish yellow to greenish yellow, etc.

There is no particular limitation on whether a yellow ink, a magenta ink or a cyan ink is selected as the color ink (A) that is one of the two color inks having the above-mentioned color relationship. A magenta ink or a cyan ink is particularly suitable for allowing development of black of high density required for text and lines In this case, the color ink (B) may be a greenish ink whose hue angle is different from that of the magenta ink by about 120° to about 240° or a reddish ink whose hue angle is different from that of the cyan ink by about 120° to about 240°. In this way, an achromatic color of dark gray can be favorably developed, and the range of color that is developed can also be broadened. The color ink (A) and color ink (B) can be overprinted on each other in either order.

Each ink constituting the ink combination for ink-jet recording contains at least water, at least one coloring agent and at least one water-soluble organic solvent.

The coloring agent may be either a water-soluble dye or a pigment. A water-soluble dye, in particular, provides excellent color developing properties and fixing properties, even when a glossy paper is used as a recording material. A single coloring agent or a combination of one or more than one coloring agent may be used in each ink.

Suitable water-soluble dyes for use in the color ink include, but are not limited to, C. I. Acid Yellow 11, 17, 23, 25, 29, 42, 61, 71, 79, 99, 110, 207, etc.; C. I. Acid Orange 7, 19, 56, 94, etc.; C. I. Acid Red 1, 6, 32, 37, 51, 52, 80, 85, 87, 92, 94, 109, 115, 180, 227, 249, 254, 256, 289, 315, 317, etc.; C. I. Acid Blue 9, 22, 40, 59, 93, 102, 104, 112, 117, 120, 167, 229, 234, etc.; C. I. Acid Green 3, 15, 25, etc.; C. I. Acid Violet 49, etc.; C. I. Direct Yellow 12, 24, 26, 86, 98, 132, 142, etc.; C. I. Direct Orange 34, 39, 44, 46, 60, etc.; C. I. Direct Red 1, 4, 17, 28, 80, 83, 92, 227, 289, etc.; C. I. Direct Violet 47, 48, etc.; C. I. Direct Blue 6, 22, 25, 71, 86, 87, 90, 106, 199, etc.; C. I. Direct Brown 109, etc.; C. I. Direct Green 59, etc.; C. I. Reactive Red 23, 31, 180, etc.; C. I. Basic Blue 1, 3, 5, 7, 9, 24, 25, 26, 28, 29, etc.; C. I. Basic Red 1, 2, 9, 12, 13, 14, 37, etc.; C. I. Basic Violet 7, 14, 27, etc.; C. I. Food Yellow 3, etc.; and the like.

Suitable pigments for use in the color ink include, but are not limited to, C. I. Pigment Yellow 1, 2, 3, 13, 16, 74, 83, 93, 128, 134, 144, etc.; C. I. Pigment Red 5, 7, 12, 23, 48 (Mn), 57 (Ca), 112, 122, 144, 170, 177, 221, 254, 264, etc.; C. I. Pigment Violet 19, 48 (Ca), etc.; C.I. Pigment Blue 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:4, 15:5, 15:6, 16, 17:1, 22, 27, 28, 29, 36, 60, etc.; C. I. Pigment Green 7, etc.; and the like.

When the coloring agent is a water-soluble dye, the solid amount of the dye can range, for example, from about 0.1 to about 20% by weight, from about 0.5 to about 10% by weight or from about 1 to about 5% by weight of the total amount of the ink. Generally the color developing property is poor when the amount of the coloring agent is less than about 0.1% by weight. On the other hand, when the color agent exceeds about 20% by weight, clogging of the nozzle of the head for ink-jet recording or the like and precipitating of the water-soluble dye can occur.

When the coloring agent is a pigment, the solid amount of the pigment can range, for example, from about 1 to about 15% by weight or from about 1 to about 10% by weight of the total amount of the ink. Generally the color developing property is poor when the pigment amount is less than about 1% by weight. On the other hand, when the pigment amount exceeds about 15% by weight, clogging of the nozzle of the head for ink-jet recording or the like and aggregating of the pigment molecules can occur.

Suitable water-soluble organic solvents useful in each ink include, but are not limited to, polyhydric alcohol alkyl ether having low odor and low vapor pressure. Such a polyhydric alcohol alkyl ether serves as a penetrant that effectively accelerates the penetration speed of the ink into a recording material such as a recording paper. Accordingly, the drying property of the ink on a recording material can be improved, bleeding (blurring at the boundary between different colors) resulting from the slow drying property on the recording material can be prevented, and feathering (whisker-shaped blurring caused by the presence of fiber and voids in a recording member) resulting from the penetration can be made less likely to occur.

Specific examples of the polyhydric alcohol alkyl ether include, but are not limited to, diethylene glycol methyl ether, diethylene glycol butyl ether, diethylene glycol isobutyl ether, dipropylene glycol methyl ether, dipropylene glycol propyl ether, dipropylene glycol isopropyl ether, dipropylene glycol butyl ether, triethylene glycol methyl ether, triethylene glycol propyl ether, triethylene glycol butyl ether, tripropylene glycol methyl ether, tripropylene glycol butyl ether and the like.

The amount of the polyhydric alcohol alkyl ether can range, for example, from about 0.3 to about 15% by weight or from about 0.4 to about 10% by weight of the total amount of each ink. When the amount is less than about 0.3% by weight, the penetration speed of the ink into a recording material such as a recording paper is slow, possibly affecting drying time and causing blurring. On the other hand, when the amount exceeds about 15% by weight, the penetration speed of the ink into a recording material is too fast, possibly causing the ink to penetrate to the back side of a recording material or causing blurring.

Each ink can contain a humectant as the water-soluble organic solvent in order to avoid clogging of the nozzle of the ink-jet head. Examples of the humectant include, but are not limited to, water-soluble glycols such as glycerin, ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, 1,5-pentanediol, 1,6-hexanediol, etc.

When the amount of the water-soluble organic solvent as the humectant is too small, it may not prevent clogging of the nozzle of the ink-jet head, and when the amount is too much, the viscosity of the ink may increase, possibly resulting in difficulty in discharging the ink from the nozzle of the ink-jet head. Typically, therefore, the amount can range, for example, from about 5 to about 50% by weight, from about 10 to about 40% by weight or from about 15 to about 35% by weight of the total amount of each ink.

The water-soluble organic solvent for controlling the penetration of the ink into a recording material such as a recording paper and the drying property of the ink, may also utilize a monohydric alcohol such as ethanol or isopropyl alcohol in each ink.

Water is used in each ink, typically one having a high purity such as ion exchanged water or distilled water. The amount of water can range, for example, from about 10 to about 98% by weight, from about 30 to about 97% by weight or from about 40 to about 95% by weight of the total amount of each ink. When the amount is less than about 10% by weight, the viscosity of the ink increases, whereby ink jetting is liable to be difficult. On the other hand, when the amount exceeds about 98% by weight, the coloring agent may precipitate or aggregate due to evaporation of water, which may lead to clogging of the nozzle of the ink-jet head.

In addition to the respective components described above, it is also possible to add any of a dispersing agent, a viscosity-adjusting agent, a surface tension-adjusting agent, a pH-adjusting agent, an antiseptic and antifungal agent and the like to each ink as needed.

The water-based ink combination for ink-jet recording can be applied to an ink-jet recording method such as an electrostatic attraction method, a method using a piezoelectric element, a thermal method or the like.

The ink-jet recording method comprises forming a black image of a low lightness to a high lightness by printing the image with the water-based ink combination for ink-jet recording such that the color ink (A) and the color ink (B) are overprinted on each other. Conventional ink-jet recording material and an ink-jet head can be used.

When a black image of a low lightness to a high lightness is formed according to the ink-jet recording method, a gray gradation image of high quality with reduced graininess and a sharp image of high density can be formed.

EXAMPLES 1 TO 4 AND COMPARATIVE EXAMPLES 1 TO 3

(1) Preparation of Inks

A blue ink (B1) was prepared as follows to have the ink composition shown in Table 1. First, 76.8 parts by weight of water, 18.0 parts by weight of glycerin, 2.0 parts by weight of dipropylene glycol propyl ether were mixed thereby preparing 96.8 parts by weight of an ink solvent. Then, while stirring this ink solvent, 3.2 parts by weight of C. I. Acid Blue 112 was added thereto, and the mixture was stirred for an additional 30 minutes. Then, the mixture was filtered through a membrane filter with a pore size of 2.5 μm, whereby a blue ink (B1) was obtained.

In the same manner as the blue ink (B1), a red ink (R1), green inks (G1 and G2), yellow inks (Y1 and Y2), an orange ink (O1), a magenta ink (M1) and a cyan ink (C1) were prepared.

(2) Measurement of L*, a*, b*, C* and h of the Color of Each Ink

Each of the prepared inks were filled in an ink cartridge, respectively, and installed in a multifunction printer with inkjet printer (MFC-3100C manufactured by Brother Industries, Ltd.). Then, printing was carried out on a recording paper (dedicated paper, glossy photo paper, BP60GLA, manufactured by Brother Industries, Ltd.) so as to cover 100% of the area of 1200×1200 dpi resolution.

According to JIS Z 8729, the lightness index (L*), the chromaticness indices (a* and b*) of the obtained printed materials were measured with a spectrocolorimeter (SC-T manufactured by Suga Test Instrument Co., Ltd.) (light source: D₆₅, viewing angle: 2°). Further, the chroma C* and the hue angle h were obtained using the following equations (1) and (2).

$\begin{matrix} {C^{*} = \sqrt{\left( a^{*} \right)^{2} + \left( b^{*} \right)^{2}}} & (1) \\ \begin{matrix} {{{{when}\mspace{14mu} a^{*}} \geq {0\mspace{14mu}{and}\mspace{14mu} b^{*}} \geq 0}:} & {h = {\tan^{- 1}\left( \frac{b^{*}}{a^{*}} \right)}} \\ {{{{when}\mspace{14mu} a^{*}} \geq {0\mspace{14mu}{and}\mspace{14mu} b^{*}} < 0}:} & {h = {360 + {\tan^{- 1}\left( \frac{b^{*}}{a^{*}} \right)}}} \\ {{{{when}\mspace{14mu} a^{*}} < 0}:} & {h = {180 + {\tan^{- 1}\left( \frac{b^{*}}{a^{*}} \right)}}} \end{matrix} & (2) \end{matrix}$

The measurement results are shown in Table 1.

TABLE 1 B1 R1 G1 G2 Y1 Y2 O1 M1 C1 Ink C.I. Acid Blue 112 3.2 composition C.I. Direct Red 80 3.0 (% by weight) C.I. Acid Green 25 3.0 C.I. Acid Green 15 3.0 C.I. Direct Yellow 86 2.4 C.I. Direct Yellow 132 2.4 C.I. Acid Orange 56 3.0 C.I. Acid Red 249 2.6 C.I. Direct Blue 199 2.5 Glycerin 18.0 25.0 30.0 19.0 25.0 20.0 32.0 24.0 26.0 Dipropylene glycol propyl 2.0 1.5 1.5 2.0 ether Triethylene glycol butyl ether 4.0 4.0 4.0 4.0 Triethylene glycol propyl ether 0.5 0.4 Ultrapure water 76.8 71.5 65.5 74.0 72.2 73.6 63.5 69.4 69.5 Measured value L* 49 48 56 57 85 88 59 52 56 a* 0 76 −25 −52 6 −10 48 80 −28 b* −64 21 −20 −32 94 102 85 −6 −51 h/° 270 16 220 212 86 96 61 356 242 C* 64 79 32 61 94 102 98 81 58 (3) Preparation of Ink Sets

The inks shown in Table 1 were combined as shown in Table 2, and ink sets of Examples 1 to 4 and Comparative Examples 1 to 3 were prepared.

(4) Hue of Black Image

Each ink set was installed in a multifunction printer with inkjet printer (MFC-3100C manufactured by Brother Industries, Ltd). Then, a patch pattern was printed on a paper for evaluation (dedicated paper, glossy photo paper, BP60GLA, manufactured by Brother Industries, Ltd.) so as to cover 100% of the area of 1200×1200 dpi resolution.

The patch pattern was printed with a printer driver capable of printing by altering the ratio of the color ink (A) to the color ink (B) to 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2 and 9:1. However, as for the ink set of Comparative Example 1, the patch pattern was printed with a printer driver capable of printing by adjusting the respective ratios of three color inks (A), (B) and (C) at ten stages.

As for the obtained printed materials, by using a spectrocolorimeter (SC-T manufactured by Suga Test Instruments Co., Ltd.), the color with an L* value of the patch patterns of the respective Examples and Comparative Examples in the range from 20 to 70 was measured (light source: D₆₅, viewing angle: 2°), and the chroma (C*) and the hue angle (h) were calculated from the obtained measurement values in the same manner as described above. Then, from the value of the chroma (C*), the hue of a black image of a medium lightness to a high lightness (medium color to light color) were evaluated in four grades based on the following criteria. The results are shown in Table 2.

-   -   A: C*≦15     -   B: 15<C*≦30     -   C: 30<C*≦50     -   D: 50<C*

In this evaluation, when the hue is evaluated as A or B, a black image of a low lightness to a high lightness can be favorably developed, when the hue is evaluated as C, a color is rendered and a black image cannot be favorably developed, and when the hue is evaluated as D, a black image cannot be developed.

(5) Evaluation of Blurring

In the same manner as in (4), each ink set was installed in the multifunction printer with inkjet printer. Then, a line was printed on a paper for evaluation (dedicated paper, glossy photo paper, BP60GLA, manufactured by Brother Industries, Ltd.), and its Rag was measured and evaluated in four grades based on the following criteria. The results are shown in Table 2.

-   A: Rag<3 -   B: 3≦Rag<5 -   C: 5≦Rag<10 -   D: 10≦Rag

Here, Rag is the raggedness of a line defined in ISO 13660. A ragged line means a line in a state that the line undulates with respect to an ideal line edge that should be smooth and straight by nature.

(6) Evaluation of Drying Property

In the same manner as in (4), each ink set was installed in the multifunction printer with inkjet printer. Then, a solid color of a 100% duty was printed on a paper for evaluation (dedicated paper, premium high quality paper, BP60PA, manufactured by Brother Industries, Ltd.). The surface of the paper for evaluation was rubbed with a finger 3 minutes after the printing, and the degree of the detachment of the coloring agent was visually observed. The drying property was evaluated in four grades as follows based on the degree of the detachment. The results are shown in Table 2.

-   A: No detachment of the coloring agent is observed. -   B: Slight detachment of the coloring agent is observed (less than     20% of the total coloring agent). -   C: Some detachment of the coloring agent is observed (20% or more     and less than 40% of the total coloring agent). -   D: Significant detachment of the coloring agent is observed (40% or     more of the total coloring agent).

TABLE 2 Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 4 Example 1 Example 2 Example 3 Ink set Color ink (A) C1 G2 G1 B1 Y1 Y2 Y2 Color ink (B) O1 R1 O1 O1 B1 G2 M1 Color ink (C) — — — — O1 — — |α − β|/° 181 196 159 209 184 116 260 Evaluation L* 40 22 59 40 25 70 45 a* −8 15 0 2 14 −35 72 b* 18 −24 14 15 −5 51 52 h/° 114 302 89 81 160 127 36 C* 20 28 14 15 15 62 89 Hue B B A A A D D Blurring A A B A D B B Drying property A A A B D B A

From the results of Table 2, it is found that a more realistic black image with a high optical density can be obtained by combining the two color inks (A) and (B) having a color relationship satisfying expression (I) above in the ink sets of Examples 1 to 4. Since a black image is formed with the two color inks (A) and (B) in the ink sets of Examples 1 to 4, the necessary amount of ink is reduced compared with Comparative Example 1 in which a black image is formed with three primary color inks, and they are excellent in terms of blurring and drying property.

Further, when an ink set including four colors in which the color ink (A) is any of a yellow ink, a magenta ink and a cyan ink, and the color ink (B) is added thereto is used, a favorable medium color can be developed compared with a three primary color ink set including a yellow ink, a magenta ink and a cyan ink, and an excellent image reproducibility and a broad color reproducibility can be realized.

On the other hand, as for the ink sets of Comparative Examples 2 and 3, it is found that since the combined two color inks (A) and (B) do not satisfy the color relationship of expression (I), a black image cannot be developed.

The water-based ink set for ink-jet recording is useful for ink-jet recording for forming a high quality gradation image in which the graininess is reduced in a black image of a low lightness to a high lightness and a full-color image excellent in color reproducibility.

While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention as set forth in the appended claims. 

1. A water-based ink combination for ink jet recording comprising a color ink (A) and a color ink (B) each of which independently has a lightness index L* according to the L* a* b* colorimetric system of about 60 or less, wherein the color ink (A) and the color ink (B) satisfy a color relationship of the following expression (I): about 120°≦|α−β|≦ about 240°  (I) wherein α is the hue angle of the color ink (A) and β is the hue angle of the color ink (B), and wherein the chroma C* of a hue obtained by the combination of the color ink (A) and the color ink (B) is about 30 or less; and wherein the color ink (A) is a magenta ink or a cyan ink, and the color ink (B) is a greenish ink or a reddish ink.
 2. The water-based ink combination according to claim 1 wherein the color ink (A) and the color ink (B) satisfy a color relationship of the following expression (II): about 150°≦|α−β|≦ about 220°  (II).
 3. The water-based ink combination according to claim 1 wherein each of the color ink (A) and the color ink (B) independently has a lightness index L* of about 30 or more.
 4. A water-based ink set for ink jet recording comprising a color ink (A) and a color ink (B) each of which independently has a lightness index L* according to the L* a* b* colorimetric system of about 60 or less, wherein the color ink (A) and the color ink (B) satisfy a color relationship of the following expression (I): about 120°≦|α−β|≦ about 240°  (I) wherein α is the hue angle of the color ink (A) and β is the hue angle of the color ink (B), and wherein the chroma C* of a hue obtained by the combination of the color ink (A) and the color ink (B) is about 30 or less; and wherein the color ink (A) is a magenta ink or a cyan ink, and the color ink (B) is a greenish ink or a reddish ink.
 5. The water-based ink set according to claim 4 wherein the color ink (A) and the color ink (B) satisfy a color relationship of the following expression (II): about 150°≦|α−β|≦ about 220°  (II).
 6. The water-based ink set according to claim 4 wherein each of the color ink (A) and the color ink (B) independently has a lightness index L* of about 30 or more.
 7. An ink-jet recording method for forming a black image of a low lightness to a high lightness, comprising printing the image by overprinting a water-based color ink (A) and a water-based color ink (B), wherein each of the color ink (A) and color ink (B) independently has a lightness index L* according to the L* a* b* colorimetric system of about 60 or less, wherein the color ink (A) and the color ink (B) satisfy a color relationship of the following expression (I): about 120°≦|α−β|≦ about 240°  (I) wherein α is the hue angle of the color ink (A) and β is the hue angle of the color ink (B), and wherein the chroma C* of a hue obtained by the combination of the color ink (A) and the color ink (B) is about 30 or less.
 8. The ink jet recording method according to claim 7 wherein the color ink (A) and the color ink (B) satisfy a color relationship of the following expression (II): about 150°≦|α−β|≦ about 220°  (II).
 9. The ink-jet recording method of claim 7 wherein each of the color ink (A) and the color ink (B) independently has an index L* of about 30 or more.
 10. The ink jet recording method of claim 7 comprising at least one of a yellow ink, a magenta ink and a cyan ink, one of which is the color ink (A).
 11. The ink jet recording method according to claim 10 wherein the color ink (A) is a magenta ink or a cyan ink.
 12. The ink-jet recording method according to claim 11 wherein the color ink (B) is a greenish ink or a reddish ink.
 13. An ink-jet recording apparatus comprising ink cartridge receivers for receiving designated ink cartridges and nozzles for application of the inks to a recording material, wherein the cartridge receivers receive at least a color ink (A) and a color ink (B) each of which independently has a lightness index L* according to the L* a* b* colorimetric system of about 60 or less, wherein the color ink (A) and the color ink (B) satisfy a color relationship of the following expression (I): about 120°≦|α−β|≦ about 240°  (I) wherein α is the hue angle of the color ink (A) and β is the hue angle of the color ink (B), and wherein the chroma C* of a hue obtained by the combination of the color ink (A) and the color ink (B) is about 30 or less; and wherein the color ink (A) is a magenta ink or a cyan ink, and the color ink (B) is a greenish ink or a reddish ink.
 14. The ink-jet recording apparatus according to claim 13 wherein the color ink (A) and the color ink (B) satisfy a color relationship of the following expression (II): about 150°≦|α−β|≦ about 220°  (II).
 15. A water based ink cartridge combination comprising a first ink cartridge containing the color ink (A) and a second ink cartridge containing the color ink (B) of claim
 1. 